Merge branch 'master' of github.com:jtsiomb/pcboot

[bootcensus] / src / libc / rand.c

diff --git a/src/libc/rand.c b/src/libc/rand.c
new file mode 100644 (file)
index 0000000..de159c2
--- /dev/null
+++ b/src/libc/rand.c
@@ -0,0 +1,65 @@
+/*
+pcboot - bootable PC demo/game kernel
+Copyright (C) 2018-2019  John Tsiombikas <nuclear@member.fsf.org>
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY, without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+/* random number generator, based on this description of the algorithm
+ * used by the GNU libc: https://www.mathstat.dal.ca/~selinger/random
+ */
+#include <stdlib.h>
+#include <inttypes.h>
+
+static int init_done;
+static int32_t rng[34];
+static int32_t *ptr0, *ptr1;
+
+int rand(void)
+{
+       int res;
+
+       if(!init_done) {
+               srand(1);
+       }
+
+       *ptr1 += *ptr0;
+       res = (uint32_t)*ptr1 >> 1;
+       if(++ptr0 >= rng + 34) ptr0 = rng;
+       if(++ptr1 >= rng + 34) ptr1 = rng;
+
+       return res;
+}
+
+void srand(unsigned int seed)
+{
+       int i;
+
+       init_done = 1;
+       if(seed == 0) seed = 1;
+
+       rng[0] = seed;
+       for(i=1; i<31; i++) {
+               rng[i] = (16807 * rng[i - 1]) % RAND_MAX;
+               if(rng[i] < 0) rng[i] += RAND_MAX;
+       }
+       for(i=31; i<34; i++) {
+               rng[i] = rng[i - 31];
+       }
+       ptr0 = rng + 3;
+       ptr1 = rng + 31;
+
+       for(i=34; i<344; i++) {
+               rand();
+       }
+}